Death of a Comet: What We Learned from the Passing of ISON

Scientists were less than thankful this year on Thanksgiving Day (November 28) when they watched the famous Comet c/2012 S1, aka ISON, expire during its fiery pass by the sun. Yet seeing ISON meet its fate taught researchers about the structure and composition of the comet and gave them a clearer picture of why it broke up near the sun.

Comet ISON was a rare interloper in the inner solar system from the faraway Oort Cloud, a sphere of comets that surrounds the sun and planets about a light-year away. Its trajectory brought it within just three solar radii of the sun's surface, putting it in a class of brazen comets called "sun grazers." "We've never seen an Oort Cloud [castoff] sun-grazing comet before," Karl Battams of the Naval Research Laboratory in Washington, D.C., said during a postmortem press conference Tuesday at the American Geophysical Union meeting in San Francisco. "It was an unprecedented object."

Discovered just last September, ISON's flight toward the sun prompted an observing campaign grander than any comet watch before. Telescopes all over the world as well as 13 space observatories and hundreds of amateur astronomers trained their sights on ISON over the past several months.

Whether ISON would survive its solar encounter to become a naked-eye spectacle was an open question. Ultimately, the comet broke up under the intense heat and tidal forces near the sun, disappointing stargazers who had hoped to catch a view of what some predicted would be the "comet of the century." By watching ISON's death unfold, researchers say they understand more about why it collapsed.

One of the bitterest twists in ISON's end was that it briefly appeared to brighten after moving beyond the sun, raising false hopes that it had survived its closest pass, known as perihelion. By carefully analyzing satellite videos, scientists now have a theory for why that happened. "One possible explanation is that fragmentation occurred before the nucleus reached perihelion," said Geraint Jones of University College London. If it broke up early, its pieces would have spread out because of the tidal forces near the sun: the closer pieces would be pulled more strongly by gravity, luring them farther in, whereas the pieces on the back side of the clump would lag due to a lessened gravitational pull. Coming out the other side of the sun, the same process would happen in reverse. When the cloud briefly coalesced again, the clump might have appeared to rebrighten before dimming out again.

One reason Comet ISON was thought to have a chance of survival was because a previous sun-grazing comet called C/2011 W3 (Lovejoy) did withstand an even closer pass by the sun in 2011. By comparing observations of both comets, scientists have theorized about what Lovejoy had that ISON lacked. "Comet Lovejoy was not an Oort Cloud comet," Battams said. "It had been past the sun at least a couple times. It had perhaps built up a thick skin. Maybe that’s a factor." ISON's outer layer was raw and possibly volatile whereas Lovejoy had been burnt by the sun and could have had a hardier crust to withstand the solar heat and pull, scientists said.

Another factor might be ISON's size. Determining the width of its nucleus is difficult, but the best estimates come from observations made by NASA's Mars Reconnaissance Orbiter (MRO) when ISON made its closest pass by Mars in October. Based on these observations, researchers think ISON's nucleus was probably smaller than 600 meters in diameter, making it relatively compact, "so its breakup and evaporation is not surprising," said Alfred McEwen of the University of Arizona in Tucson, principal investigator for the HiRISE (High-Resolution Imaging Science Experiment) camera on the MRO.

More than a month after zooming by Mars, ISON passed near Mercury, where NASA's MESSENGER probe stationed in orbit around the planet snapped thousands of spectroscopy measurements, which broke up the comet's light into its constituent wavelengths to analyze the object's composition. The scans revealed signatures of carbon, oxygen, sulfur, sodium and other elements present in ISON's nucleus. "It's actually a very rich data set and we've only just started to look through that," said MESSENGER project scientist Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory in Maryland. "The major thing of interest is how many carbon lines that one actually sees in these spectra. There are some spectral structures that have not been identified yet."

Although ISON is dead, it could still have lessons to teach. The Hubble, Spitzer and Chandra space telescope teams are all considering making observations to look for dust left over from the wrecked comet. "That’s sort of a recovery mission, but I don’t know if we're going to get any success with those," Battams said.

And the Comet ISON Observing Campaign was the perfect training for observing another noteworthy comet's approach. In 2014 an Oort Cloud comet called C/2013 A1 (Siding Spring) is due to arrive in the inner solar system; it will fly extremely close to Mars, where MRO will again take aim. MRO's HiRISE camera should be able to resolve a higher-resolution image of Siding Spring than those of ISON, which were blurry due to distance. "This will be our first resolved image of a long-period comet," McEwen said. "This will help answer important questions about how long-period comets differ form short-period comets. So stay tuned for that next year."